Honey, They've Shrunk the Rover
The Sojourner vehicle that trekked across the Martian surface captured the world’s fancy last summer, but we haven’t seen anything yet. Next will come rovers that can roam miles across Mars and “aerobots” able to survey other planets.
If the Nobel judges gave a prize for over-achievement, a good candidate might be a six-wheeled, breadbox-sized robot named Sojourner. As robots go, the rover that explored Mars last year was a modest contraption; it traveled only about two feet a minute and performed just two simple scientific operations, photographing rocks and reading their chemical signatures. And the machine cost just $25 million, with the Pathfinder mission that carried it to Mars a bargain at $266 million, a quarter the cost of a single space-shuttle flight. But Sojourner was the Energizer Bunny of robots, the rover that just kept on roving. Officials cautiously projected that it might operate for a month on Mars; it sent back data for three.
It also seized the public imagination as no space venture had since Neil Armstrong and Buzz Aldrin planted the United States flag on the moon. In the mission’s first heady weeks, NASA’s multiple Pathfinder World Wide Web sites recorded about 45 million hits a day. Time and Newsweek accorded Pathfinder’s July 4 landing and Sojourner’s subsequent debarkation simultaneous covers and the sorts of spreads usually reserved for the starts of wars or the deaths of princesses. Mattel immediately sold out the first run of its Sojourner “Mighty Wheels” toy. At a midsummer reception, Vice President Al Gore joked that he had “been replaced by Sojourner as the world’s favorite robot.” Had humankind ever identified so closely with
Thus did the rover and its faithful lander save NASA from the stigma of two decades of costly boondoggles, fatal disasters, and dashed expectations. The essential strategy of the reformed NASA-nix the costly, dangerous manned expeditions and let robots and other remote exploration tools do the work-had panned out.
That was just the start. NASA is now preparing succeeding generations of planetary explorers that will make Sojourner look decidedly humdrum: rovers to traverse many miles of Mars’s expanses, gathering far-flung samples for transport back to Earth; “penetrators” to probe the living worlds that might lie beneath extraterrestrial rock and ice crusts; and “aerobots” to survey other planets and their moons, from Venus to Jupiter’s moon Titan, and perhaps even Uranus and Neptune, from the air.
As exotic and wildly varied as these devices may seem, they all derive from the same critical change in NASA’s approach to unmanned planetary exploration nearly nine years ago-“faster, cheaper, better.” This change was initiated not through official NASA policy but in defiance of it, by a small group of robot-building heretics who saw a better way to go to Mars than their bosses saw and who worked covertly to make the idea possible. Appreciating the roots of that robot-building rebellion helps in understanding why planetary exploration is now taking the course it is.
In the Beginning
The philosophy underlying “faster, cheaper, better” is “less is more.” In space flight, mass equals money-lots of it-and complexity means risk. To make, say, rovers affordable and reliable, create them to be as lightweight and simple as possible-25 pounds in Sojourner’s case, with a feeble but sturdy 8-bit, 1970s-level processing unit for a brain. As obvious as this strategy seems now, through the late 1980s NASA’s Jet Propulsion Laboratory (JPL) focused on building something completely different: a truly formidable machine in size, range, and computational and operational ability-and cost. This Mars Rover Sampler Return (MRSR) vehicle would measure up to 8 feet long and weigh half a ton. It would be built to cruise for a year and a half in Mars’s punishing cold, across 700 miles of its varied, rugged terrains, gathering samples and blazing a path for human missions. It would cost up to $10 billion.
“Then fiscal reality hit,” recalls David Lavery, manager of NASA’s telerobotics research program. “We realized it just wasn’t going to happen.”
Luckily, unbeknownst to the NASA brass, an alternative waited in the back laboratories at JPL-a neglected experiment in robot automation and simplification. Even as JPL as an institution was chasing the MRSR dream to its dead end, a small group of upstart rover and another group of artificial-intelligence renegades, both at JPL, had been quietly seeking solutions of their own.
The effort began in 1988, when Howard Eisen, now chief mobility engineer for JPL’s rovers, left his graduate studies at MIT to work at the lab. He brought with him a particularly apt thesis project: building a one-eighth-scale model of the mighty MRSR. He found that the model, guided by an electric tether, performed much better than expected. Indeed, its five-inch wheels could climb over objects up to eight inches high. So perhaps the jumbo MRSR was unnecessary, he thought; could “a much smaller platform” negotiate Mars’s rocky surface?
Eisen and his JPL colleagues set out to build such a platform-working on their own in the garage of engineer (and former hot-rodder) Don Bickler. After several trials Bickler invented a six-wheeled chassis that could maintain even weight and traction on all its wheels. The engineers called it a “rocker/bogey” after its two key mechanical elements and with all due honor named the subsequent rover prototypes Rocky. “Everyone was joking over whether we’d have as many sequels as the Rocky movies,” Eisen recalls. (They would.)
Midway through building the first Rocky, the rover renegades obtained lab space at JPL and gained new collaborators: a team of artificial-intelligence (AI) designers. Inspired by the “subsumption architecture” approach of MIT’s AI pioneer Rodney Brooks-designing robots to operate using a hierarchy of simple reactions to stimuli-David Miller, a recent arrival at JPL’s AI section, hired Brooks’s student Colin Angle for a summer. At MIT Angle had created the trailblazing Genghis robot, which despite its low brainpower could autonomously perform a fairly complex function-gathering up all the coffee cups in the office. (That machine is now in the National Air and Space Museum.) At JPL, Angle built a similar robot named “Tooth” using a model-car chassis, for less than $500 in parts and $5,000 in labor. “My only constraint,” Angle recalls, “was I couldn’t spend more than $50 for each part, so it could all come out of petty cash.”
Miller and his teammates saw the potential of the mechanical Rocky that Bickler’s group had developed. Marrying Tooth’s electronic “brain” to Rocky 3’s body, they created the first autonomous rover that could operate outdoors, on actual dirt.
JPL managers were impressed, but still wedded to their big MRSR. Then Congressional members howled at its cost and that project was dead. NASA had meanwhile found funding for a single small Pathfinder lander and cast about for something to carry on it. “I said, We happen to have this one rover,’ ” recalls Miller. At last the renegades had an actual mission to work on.
But not for long. As so often happens when innovations are channeled into the mainstream, the innovators were left out in the cold. Miller and his teammates lost control of the Rocky project, and all but one of them left JPL for the private sector. Rover budgets increased, and so did timelines. Miller’s group took just one-and-a-half years to progress from Tooth to the fourth-generation Rocky, but JPL then took five more years to advance to Sojourner, the sixth in the Rocky line.
JPL’s Brian Wilcox, who took the helm of the microrover project after Miller, argues that this was a natural response to the challenges of making technologies “reliable enough” to work in the harsh Martian environment. Indeed, novel as it seemed to TV viewers, Pathfinder was a rather conservative mission; JPL reached back to tried-and-true command technology rather than attempting fully autonomous operation. Better safe than venturesome when the whole world is watching.
But now that Sojourner has paved the way, other planetary explorers can realize the potential it only hinted at. Rocky 7-outfitted with a robot arm and camera mast as well as a Sojourner-style chassis-is going through its paces at dry Lavic Lake in the Mojave Desert. The lakebed is an especially apt Mars analog, thanks to the aviators at the nearby Twentynine Palms Marine base; their bombing practice has left it pocked with craters, approximating the thousands that asteroids have nicked in Mars’s surface.
The long-distance hiking that Rocky 7 is practicing under autonomous operation and with improved sensory and navigation systems will be critical for future Mars rovers. Sojourner crawled only a few score meters under the watchful, position-checking electronic eye of its Pathfinder mother ship. But Rocky 8, the rover-apparent for the 2001 Mars mission and its 2003 successor, will have to cover many miles, probably on rougher, older highland terrain where traces of ancient life may more likely be found. NASA wants these rovers to cache interesting samples that yet a third machine-a brawnier, more specialized “retrieval rover”-is slated to pick up in 2005 with the aid of electronic beacons left with the caches. That rover might have to haul the samples still more miles to the lander for return to Earth and the close examination that may finally settle the question of life on Mars.
Covering more ground is just one of many challenges to come for the rover makers. Others revolve around more restricted budgets. Steve Saunders, JPL’s chief project scientist for the Mars 2001 mission, notes that future vehicles will have to rely on less human involvement. While the Pathfinder mission occupied up to 10 people several months after the landing with matters such as control and troubleshooting, the Mars 2001 budget allows for an operations team of about 4. The next rovers also must cost less, ride on smaller rockets, do more science (NASA is still hashing out exactly what) and run 3 times longer than Sojourner. And aside from monetary factors, they must survive an even wider range of temperatures than Sojourner did (experts figure the Martian cold finally silenced the Pathfinder mission).
Much of the hope for meeting these challenges rests on the new graphite composites that JPL’s mechanical systems division is creating. Using consistent composites throughout a rover could reduce the destructive differential cooling and contracting that now occurs when various metals are used, making the machine less vulnerable to temperature changes. And composites could shave more weight off the final payload, reducing costs. Already, a prototype “Lightweight Survivable Rover” for the ‘05 mission weighs only 15 pounds-two-thirds as much as Sojourner-while stretching more than one-and-a-half times as long and wide and standing nearly twice as high, a foot off the ground.
Further, this prototype’s wheels collapse to a third of their extended volume and thus can pack into a smaller flight capsule, says Paul Schenker, the division’s research and development leader. “We’re extending that idea to the whole rover frame,” he adds, vowing to make the sample-retrieval rover “truly collapsible” and hence even cheaper to send.
The greatest progress by Schenker’s team has been in building arms from the lightweight composites. One arm, all composite down to its motor, weighs only about eight pounds but can lengthen to about six feet, dig a trench, lift and deposit samples, and sling a microcamera. Another, weighing two pounds, can lift several times its own weight, in part due to ultrasonic motors (so called because they whir at inaudible frequencies). Such motors maximize torque-hence traction and leverage-at very low speeds, which is just what you want for extraterrestrial uses, where high speeds increase the risk of accidents and require more information processing. Moreover, low-speed ultrasonic motors don’t require gearboxes as conventional motors do to reduce their rotations to useful speeds. Eliminating the gearbox eliminates more weight; again, less is more.
By the time the ultralight rovers with boarding-house reaches are ready to fly, an even more dramatic essay in rover miniaturization may have proven itself in the first-ever landing on an asteroid. Again, necessity, in the form of payload restrictions, is the mother of design. In September 2003, the Japanese space mission known as Muses-C is due to land on the half-mile-wide, earth-crossing asteroid Nereus. The plan is to touch down at three sites (lifting off and landing again requires little power in an asteroid’s low gravity), collect samples, and dispatch these to earth by January 2006 using parachutes dropped from space flights. But first Muses-C should drop off an American passenger-a rover.
When Japan’s Institute of Space and Astronomical Science (ISAS) came to NASA seeking technical help on Muses-C, it offered NASA the chance to fill out the lander’s unused cargo space. The Americans thought of sending a complement of scientific instruments, but decided that if these were attached to the lander they would merely duplicate the Japanese effort. Better to send a rover to explore other parts of Nereus.
Just one hitch: Muses-C has only two pounds’ worth of extra cargo capacity, with half needed for computer and communication equipment to enable NASA to “talk” directly with the rover. Ergo the next step in miniaturization: a one-pound “nanorover.” (This is a term of art, since nanotechnology usually refers to work at the molecular level. But what else do you call a machine one-twentieth the size of a “microrover?”) The nanorover’s science instruments will be more sophisticated than the relatively gigantic Sojourner’s: an infrared spectrometer for reading chemical signatures by infrared rays, an imaging camera with eight-position filter wheels for reading various light spectra, and perhaps an x-ray spectrometer. But the machine will have a much simpler chassis. A current prototype has only two wheels, on which it will skid and even flip over (and then right itself). On an asteroid, where impacts are glancingly light, such careening won’t be the disaster it would be on a full-sized planet. In low gravity, where stopping is difficult, such movements are inevitable anyway.
An asteroid presents even more fearsome temperature challenges than Mars. Brian Wilcox, JPL’s robotics group supervisor, notes that Sojourner operated only when it was warm-after its gel insulation had trapped enough heat each day. But insulating is futile on something as small as a nanorover, with its proportionally high surface area. And asteroid explorers must brace for 250-degree Celsius temperature fluctuations from day to night. Electrical components are typically rated only up to an automobile’s temperature range-about 120 degrees. Finding components that can handle minus-125 degrees is a main challenge, says JPL systems engineer Rick Welch, who has worked on the nanorover. The appropriate parts tend to be CMOS-complementary metal-oxide semiconductor-electronics, which maintain conductivity and work at extremely low temperatures. The asteroid rover will be a night crawler; “during the [scorching] day, we’ll just turn it off,” he notes.
For the past four years JPL has worked on the idea of aerial exploration as well: aerobots, autonomous robotic balloons that could cover a much wider territory than any ground rover while producing much higher-resolution photography than satellites could. The idea isn’t novel; in 1985 the Soviets and French sent a survey balloon to Venus. It performed briefly but well, bobbing upward when it approached the hot Venutian surface and the gases in its bag expanded, then downward when it hit the cold stratosphere and those gases condensed.
But the aerobots JPL is designing (and for which it will launch a test bed early this year) are much more sophisticated. Rather than floating at a constant height, they will control their altitude through valves that can release or confine the gases that give them buoyancy. Thus, explains JPL aerobot systems engineer Aaron Bachelder, they will be able to hover for some time (perhaps an hour or so above Venus, because of the fierce heat, around 460 degrees Celsius on its surface). Then they’ll retreat to the stratosphere to cool off. “Snakes”-long, dangling flexible appendages-will protect against crashes by transferring weight from the aerobot to the ground if the balloon hovers too low. The aerobots will also contain streamlined sensing and scientific instruments for up-close study of the surface. And, JPL special projects manager Jim Cutts insists, they’ve actually been proven not to catch on rocks in earth trials by the French. He adds, taking a page from the nanorover play book, that his crew is designing aerobots light enough-around 22 pounds-to hitch a ride with other missions.
NASA was figuring on Venus as the first aerobot destination, since its heat-too great for ground rovers-makes that planet an optimal choice for the balloons. And Venus’s predictable winds also make it an easier place to plan routes for the devices than blustery Mars. But now that Pathfinder’s triumph has made Mars fashionable, Cutts also hopes to send an aerobot to Mars in 2003-perhaps riding along with that year’s rock-hunting rover.
Aerobots’ prospects don’t end with the two nearest planets. JPL has also sketched out aerobot missions to the Jovian moon Titan and the outer gas planets. Because those planets have much lighter atmospheres, light-gas balloons would not work on them in the way they’re expected to on solid planets. And so missions to Jupiter, Saturn, Uranus, and Neptune would rely on a different, venerable technology: hot-air balloons, heated by the planets’ own infrared radiation.
If the aerobots pan out, technology will have come full circle; balloons, the earliest form of airborne transport, will fly in the vanguard of planetary exploration. That’s just one more indication of the diversity in approaches NASA has been employing in the eight years since it stopped pinning its hopes for investigations on a single massive Mars rover.
Become an MIT Technology Review Insider for in-depth analysis and unparalleled perspective.Subscribe today